How to use blue weight paint?
Red indicates high weight.  Blue indicates low weight.  So what you're asking is, how to reduce the weight?

Simple answer:  Change your brush to subtract.  See the pic:



Information you may not want want, but should want:

You can select another bone, paint it more red, and then normalize your weights.  Increasing the weight of one bone decreases the weights of other bones.

Another way to do it is by changing the weight of a brush.  You can use mix, darken, or multiply with a low brush weight.